Discrete step rotary actuator

ABSTRACT

A discrete step rotary actuator ( 1 ) comprises a stator ( 2 ), a rotor ( 3 ) and at least an actuating element ( 10 ) made with a shape memory active material, the actuating element ( 10 ) having a first portion ( 11 ) anchored to said stator and a second portion with a terminal element ( 12 ). The rotor has a sequence of seatings ( 14 ) arranged as a circumference, into which the terminal element ( 12 ) can engage in a sequential way. Elastic means ( 13 ) placed between the actuating element ( 10 ) and the stator ( 2 ) induce the shift of the terminal element ( 12 ) between two consecutive seatings ( 14 ), during the passage of the active material from its shortened to its extended configuration. The passage of the active material from the extended configuration to its shortened configuration imparts the rotor ( 3 ) a rotation couple with respect to the stator ( 2 ).

This is a National Stage entry of Application PCT/IB2004/000648, with aninternational filing date of Feb. 23, 2004, which was published underPCT Article 21(2) as WO 2004/082108 A1, and the complete disclosure ofwhich is incorporated into this application by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a discrete step rotary actuator,comprising a stationary portion or stator, a rotary portion or rotor andmeans for rotating the rotor with respect to the stator.

Electric actuators of the type referred to, also known as step-by-stepmotors or steppers, are used in applications requiring an accurateangular shift of the rotor.

The operation of such actuators or motors, usually with a permanentmagnets, with variable reluctance or of hybrid type, provides that aseries of current pulses, according to a given sequence, is sent to theactuator, so as to shift the rotor by consecutive jogs, until a balanceposition is obtained. It is thus possible to rotate a shaft associatedwith the rotor in the desired position and at the desired speed, bysimply counting pulsing and setting their frequency, since the balancepositions of shaft and rotor are determined mechanically with a highaccuracy.

Known step-by-step actuators are usually bulky and require complexelectronic driving circuits, generally digital circuits. The typicaljogging operation of said actuators further results in vibrations andnoise, above all at low speeds and when simpler driving techniques areused.

SUMMARY OF THE INVENTION

The present invention aims at carrying out a new discrete step rotaryactuator having a small size, a high power in relation to size, anoiseless operation and not requiring complex control systems ormechanical reducers.

In view of achieving said aim, the object of the invention is a discretestep rotary actuator as specified above, characterized in that the meansfor rotating the rotor with respect to the stator comprise:

-   -   at least an actuating element made at least partly with a shape        memory active material, which can take a shortened configuration        and an extended configuration, the actuating element having a        first portion anchored to one of said stator or rotor,    -   a sequence of seatings arranged as a circumference around the        other one of said rotor or stator, the actuating element having        a second portion that can engage said seatings sequentially,    -   elastic means placed between the actuating element and the one        of said stator or rotor to which said first portion of the        actuating element is anchored.

Said elastic means are operative for inducing a shift of said secondportion of the actuating element between two consecutive seatings ofsaid sequence, during the passage of the active material from itsshortened to its extended configuration, whereas the passage of theactive material from the extended configuration to its shortenedconfiguration imparts the rotor a rotation couple with respect to thestator.

In a preferred embodiment, the active material used is a shape memorymetal alloy. Actuators made of a shape memory alloy or SMA are known perse and have already been used for several applications in varioustechnical fields. They generally use at least an element made of a metalalloy that can change its structure beyond a given transitiontemperature. In other possible embodiments, the active material used tomake the actuating means of the actuator according to the invention canbe a shape memory polymer or SMP or an electro-active polymer or EAP.

The actuating element or elements provided for are preferablythread-shaped and can be connected to electric supply means, which heatup said elements by Joule effect above the transition temperature of theactive material. In case of several actuating elements, the latter canbe supplied simultaneously or sequentially; several groups of actuatingelements can also be supplied sequentially.

Alternatively, the actuating element or elements are driven directly bymeans of the temperature of a fluid in which the actuator is operating,for instance a gas or a liquid.

Thanks to the characteristics referred to above, the actuator accordingto the invention has a simple structure, a small size and a low cost,while its operation is accurate, noiseless and can be easily controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the invention will be evidentfrom the following description referring to the accompanying drawings,provided as mere non-limiting examples, in which:

FIG. 1 is a schematic view in elevation of a rotary actuator accordingto the invention, in a first operating condition;

FIG. 2 is schematic view in elevation of a portion of the rotaryactuator of FIG. 1, in a second operating condition;

FIG. 3 is a schematic view resembling the one in FIG. 2, with the rotaryactuator in a third operating condition;

FIG. 4 is a perspective view, partially sectioned, of a portion of therotary actuator according to a possible embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the figures number 1 globally refers to a rotary actuator accordingto the invention, which comprises a stationary portion or stator,referred to with number 2, and a rotary portion or rotor, referred towith number 3. Note that in FIG. 1, as well as in the following FIGS. 2and 3, some of the components of the actuator are referred to with thenumbers mentioned below plus letters A, B, C and D, so as to better showtheir positioning variations; in the following description, however,only numbers will be mentioned.

The stator 2 has a central housing with a circular section, for instanceshaped as a through cavity, within which the rotor 3 is turnablymounted; in the case shown by way of example, the stator 2 and the rotor3 have a basically cylindrical hollow shape and are coaxial one to theother, the outer circular surface of the rotor 3 being adjacent to theinner circular surface of the stator 2. By way of example, the stator 2can have an outer diameter of 15–20 mm and a thickness of 2–3 mm.

According to the invention, in order to generate a rotation couplebetween the rotor 3 and the stator 2, one or more actuating elements areprovided for, referred to with number 10, at least partly made of anactive material, i.e. a material whose structure can be selectivelymodified by applying a stimulus from outside.

In the preferred embodiment of the invention, the active material usedis a shape memory metal alloy. As previously mentioned, such metalmaterials can recover their initial configuration if deformed and thenundergoing a suitable heat treatment. In particular, shape memory metalalloys undergo a crystalline phase modification when going from theirstiffest configuration at high temperature (austenite) to theirconfiguration at lower energy and temperature (martensite). When broughtto a low temperature, an element made of a shape memory alloy takes amartensite-like structure, with a low yield point, and is easilydeformable; after heating the alloy takes another crystalline structure,austenite-like, and then recovers its initial structure and shape. Thetransition temperature, starting from which the alloy “remembers” itsprimitive shape, can be changed by varying the composition or bysuitable heat treatments. The most interesting alloys with “shapememory” properties are those with a considerable deformation recovery orgenerating a remarkable strength during phase transition, such as Ni—Ti,Ni—Ti—Cu, Cu—Al—Zn, Cu—Al—Ni alloys, which can be used to implement theinvention.

In the case shown by way of example in FIG. 1, the actuator 1 comprisessix actuating elements 10 shaped like a thread or in any case having athin and oblong shape, all of the same length; by way of example, saidthreads 10 can have a length of about 3–5 mm.

Each thread 10 is anchored on one side to the stator 2, in acorresponding area referred to with 11; on the opposite side, eachthread 10 is associated with a terminal element 12, not necessarily madeof an active material. Each terminal element 12 is fastened to the firstend of a corresponding coil spring 13, the other end being anchored tothe stator 3. Said springs 13 are arranged so as to exert a tractiononto the corresponding terminal elements 12 in a direction substantiallyopposite the one of the anchoring point 11 of the respective thread 10,with reference to the direction of rotation of the rotor, referred towith F in FIG. 1.

As can be seen in FIG. 4, both the rotor and the stator can consist eachof two coaxial discs 2′, 3′, made integral one to the other throughmeans known per se; thus, a space between the discs 2′ and the discs 3′is defined, through which extend the threads 10 and the springs 13. FIG.4 also shows how the threads 10 can be U-shaped, with a going and areturn portion with respect to the terminal element 12, so that bothends of the thread are close to one another, thus making the electricand mechanical connection in the corresponding area 11 easier.

The inner peripheral surface of the rotor 3 is provided with equidistantengagement seatings 14 for terminal elements 12; in the case shown byway of example, and as can be seen in FIG. 4, the terminal elements 12are shaped like small cylinders and their engagement seatings have asubstantially concave shape, open in the direction opposite thedirection F of rotation of the rotor 3.

The inner peripheral surface of the rotor 3 is shaped so as to define,between two consecutive seatings 14, a slope 15 or “sky-jump”, i.e. asurface with a slightly curved development, which joins two planslocated at different heights; in particular, each slope 15 has adevelopment ascending from a seating 14 to the following one, withreference to the direction opposite the direction F of movement of therotor 3. As for the threads 10 and the springs 13, in the case shown byway of example, there are six seatings 14 and six slopes 15.

In the preferred embodiment of the invention, the ends of the threads 10are connected to an electric supply source, schematically referred towith AE in FIG. 1, so as to let an electric current go through saidthreads and then heat the latter by Joule effect. Depending on thecontrol pattern chosen for the actuator 1, the threads 10 can besupplied simultaneously or sequentially. The actuator 1 works asdescribed in the following with reference to the case of simultaneoussupply of all threads 10.

FIG. 1 shows a rest condition of the actuator 1, in which the threads 10are not supplied; as can be seen, in said condition the terminalelements 12 are arranged each in a respective engagement seating 14 ofthe rotor 3.

As a consequence of the heating due to the passage of electric current,the threads 10 get over their transition temperature and take ashortened or a small length configuration, as can be seen in FIG. 2,thus pulling the terminal elements 12 towards their respective anchoringareas 11.

When passing from their extended to their shortened configuration, thethreads 10 thus impart the rotor 3 a rotation couple, which results inthe passage of said rotor from the position indicated with a hatchedline to the one indicated with the full line in FIG. 2. It is thuspossible to obtain a discrete angular movement of the rotor 3 withrespect to the stator 2.

After reaching the position of FIG. 2, the electric supply to thethreads 10 is interrupted; thus, their temperature decreasesprogressively and sinks below the transition temperature of the shapememory alloy, so that said threads take back their respective extendedconfigurations.

The passage from the shortened to the extended configurations (as isindicated with a hatched line in FIG. 3, only for thread 10B and itsrespective terminal elements 12B and spring 13B) is helped by the actionof the springs 13, which “pull” the terminal elements 12 and make themslide on the slopes 15, until said elements engage into a seating 14following the one they previously occupied.

Thus the operating position indicated with a full line in FIG. 3 isreached, in which each terminal element 12 engages a seating 14following the one previously occupied; the position of the seatings 14indicated with a hatched line in FIG. 3 corresponds to the position ofFIG. 1.

Then the threads 10 will be supplied again, thus obtaining a newdiscrete angular movement of the rotor 3 with respect to the stator 2,and then current supply will be interrupted again. The operating cycleis repeated until the desired actuation is obtained.

The shape memory material used for the actuating elements 10 accordingto the present invention could be a non-metallic material, and inparticular a shape memory polymer or SMP. As is generally known, suchpolymers can modify their stiffness and shape depending on thetemperature they are subject to and swiftly pass, if heated, from aglass-like state to a highly deformable rubber-like state, then goingback as swiftly as before, when cooled, to their original shape andhardness. Moreover, similarly to metal alloys, thanks to their “memory”,SMPs can recover for an endless number of times their original shape, ifbrought again beyond their critical temperature.

If the threads 10 are made of a shape memory polymer, the supply cycleof said threads will be opposite with respect to the one describedbefore, i.e. with a current supply so as to obtain the passage of saidthreads from their shortened to their extended condition and,conversely, an interruption of current supply so as to let them passfrom their extended to their shortened structure; in this application,therefore, the rotation couple will be imparted to the rotor 3 when thethreads 10 pass from their extended to their shortened configuration,and thus during a step in which said threads are not supplied withelectricity.

In a further possible embodiment, the shape memory material used atleast for a portion of each actuating element 12 could be anelectro-active polymer or EAP, i.e. a polymer material that can undergodeformations if an electric field is applied to it, chosen in particularamong:

-   -   electrostrictive polymers, i.e. polymers that, when subject to        electric fields, react by reducing the size parallel to the        field and increasing the size orthogonal to said field;    -   IPMC polymers (ion polymer metal composites), i.e. metal-ion        composite polymers whose terminations can ionize into polar        liquids (one among the most used IMPCs is NAFION® by DuPont,        used as “artificial muscle”);    -   conductive polymers, i.e. polymers that change, when ionization        varies, their mechanical properties and size and can therefore        be used with great advantages both as sensitive elements and as        mechanical actuators.

The invention enables to carry out discrete step actuators withminiaturized size and high power density, which do not require reducersor complex control systems and which are noiseless and accurate. In saidlight, the applications of the actuator 1 are manifold; in particular,it should be pointed out that the invention can be used with greatadvantages in the field of micro electromechanical systems or MEMS, formaking miniaturized devices such as motors, pumps, turbines, shutters,flow deflectors, etc.

It should be pointed out that, if the sequence rapidity of the discreteactuating steps should be privileged, the threads 10 could be suppliedin a sequential or phase-shifted way one with respect to the other, andnot simultaneously. Obviously, in such a case, if on one side the coupleimparted to the rotor 3 is smaller, on the other side cycle time can bereduced.

It is also evident that both suggested control techniques can beadvantageously combined, for instance by supplying sequentially pairs ortriplets of threads 10, so as to obtain a desired couple and a desiredactuating speed. In said light, it is also evident that the number ofthreads 10 can vary with respect to the number (six) previouslymentioned by way of example; for instance there could be sixteen threads10, supplied sequentially four by four.

In a possible embodiment, the threads 10 could act by detecting directlythe temperature to which they are subject, for instance the temperatureof a gas or a liquid, so as to be actuated by said temperature at atransition value that can be adjusted when preparing the active materialused; in said light, for instance, the actuator 1 could be designed tocontrol a shutter and be directly immersed in a liquid to be controlled.When said liquid passes from a first to a second given temperature, thethreads pass from their extended to their shortened configuration (orconversely, depending on the active material used), and then go back totheir initial condition when the liquid goes back to the firsttemperature, so as to obtain automatically an actuating step of therotor 3. In said application, the threads 10 could also be single-lengththreads, i.e. not U-shaped, and thus with an end anchored to thecorresponding area 11 and the other end associated with thecorresponding terminal element 12.

Obviously, though the basic idea of the invention remains the same,construction details and embodiments can widely vary with respect towhat has been described and shown by mere way of example, howeverwithout leaving the framework of the present invention.

The functions of the stator 2 could be performed by any structure havinga stationary position with respect to the rotor 3.

The functions of the components referred to with numbers 2 and 3 couldbe inverted with respect to those previously mentioned by way ofexample, for instance with the component 3 in stationary position,acting as a stator, and the component 2 making angular movement, actingas a rotor.

It is then obvious to the person skilled in the art that in a possibleembodiment with the stator in central position and the rotor inperipheral position, the engagement seatings 14 and the slopes 15 couldbe defined on the outer peripheral surface of the rotor.

The actuator previously described can rotate only in one direction but,as was said, can have an extremely small size (basically like a coin).In said light, an actuator according to the invention can be made so asto be piled up onto another actuator of the same type, though orientedwith opposite direction of rotation. Thus, by coupling two actuators,one for each direction of rotation, it is possible to obtain a compactdevice, that can be actuated in both directions of rotation.

1. Discrete step rotary actuator (1), comprising a stationary part orstator (2), a rotary part or rotor (3) and means (10, 13, 14) forrotating the rotor (3) with respect to the stator (2), characterized inthat said means (10, 13, 14) comprise at least an actuating element (10)made at least partly with a shape memory active material, which can takea shortened configuration and an extended configuration, the actuatingelement (10) having a first portion (11) anchored to one of said stator(2) and rotor (3), a consecutive sequence of consecutive seatings (14)arranged as a circumference around the other one of said rotor (3) andstator (2), the actuating element (10) having a second portion (12) thatcan engage said seatings (14) sequentially, elastic means (13) placedbetween the actuating element (10) and the one of said stator (2) androtor (3) to which said first portion (11) of the actuating element (10)is anchored, where said elastic means (13) are operative for inducing ashift of said second portion (12) of the actuating element (10) betweentwo consecutive seatings (14) of said sequence, during the passage ofthe active material from its shortened to its extended configuration,the passage of the active material from the extended configuration toits shortened configuration imparting the rotor (3) a rotation couplewith respect to the stator (2).
 2. Actuator according to claim 1,characterized in that said active material is selected in the groupconsisting of shape memory metal alloys, shape memory polymers,electro-active polymers.
 3. Actuator according to claim 1, characterizedin that said actuating element (10) can be subject to an electric orheat stimulus so as to obtain its passage from its shortened to itsextended configuration, or vice-versa.
 4. Actuator according to claim 3,characterized in that said actuating element (10) is connected toelectric supply means (AE), which are operative for heating it by Jouleeffect.
 5. Actuator according to claim 3, characterized in that saidactuating element (10) is arranged so as to be actuated by thetemperature of a fluid to which said element is subject.
 6. Actuatoraccording to claim 1, characterized in that said elastic means (13) areoperative to induce exert a traction onto the respective actuatingelement (10), in order to move said second portion (12) away from saidfirst portion (11).
 7. Actuator according to claim 6, characterized inthat said elastic means comprise a spring (13), particular a coilspring.
 8. Actuator according to claim 7, characterized in that saidsecond portion comprises a terminal element (12) with which an end ofsaid spring (13) is associated.
 9. Actuator according to claim 1,characterized in that said seatings (14) have a substantially concaveshape, whose cavity is open in the direction opposite the direction ofangular movement (F) of said rotor (3).
 10. Actuator according to claim1, characterized in that said seatings (14) are substantially at thesame distance one from the other and are arranged on a circumferentialsurface of one of said stator (2) and rotor (3).
 11. Actuator accordingto claim 10, characterized in that said circumferential surface isshaped so as to define, between pairs of subsequent seatings (14) ofsaid sequence, a slope (15) with a descending development with respectto the direction of angular movement (F) of said rotor (3).
 12. Actuatoraccording to claim 8, characterized in that said actuating element (10)has a thin and long shape.
 13. Actuator according to claim 1,characterized in that said stator (2) and rotor (3) are made up each ofa pair of substantially disc-shaped, coaxial and parallel elements (2′,3′), said actuating element (10) and the respective elastic means (13)extending at least partly in a space defined between the two disc-shapedelements (2′, 3′) of each pair.
 14. Actuator according to claim 1,characterized in that it is provided for a plurality of actuatingelements (10), each of which is associated with respective elastic means(13).
 15. Actuator according to claim 14, characterized in that it isprovided for a simultaneous electric supply of a plurality of saidactuating elements (10).
 16. Actuator according to claim 14,characterized in that said actuating elements (10) are supplied withelectric energy in a sequential way.
 17. Actuator according to claim 14,characterized in that at least a first plurality of said actuatingelements (10) is supplied sequentially with respect to a secondplurality of said actuating elements (10).
 18. Actuator according toclaim 1, characterized in that said stator (2) is in a peripheralposition with respect to said rotor (3).
 19. Actuator according to claim1, characterized in that said rotor is in a peripheral position withrespect to said stator.
 20. Actuator according to claim 10,characterized in that said circumferential surface is an inner or outerperipheral surface of said stator (2) or rotor (3).
 21. Method forgenerating a discrete step rotation of a stationary part or stator (2)with respect to a rotary part or rotor (3), comprising the followingsteps: providing out at least an actuating element (10) at least partlymade of a shape memory active material which can take a shortened and anextended configuration; making a consecutive sequence of consecutiveseatings (14) arranged on of said stator (2) or rotor (3); anchoring afirst portion (11) of the actuating element (10) to one of said rotor(3) and stator (2), so that a second portion (12) of the actuatingelement (10) engages a first of said seatings (14) placing elastic means(13) between the actuating element (10) and the one of said stator (2)or rotor (3) to which the first portion (811) of the actuating element(10) is anchored; applying an electric or heat stimulus to the activematerial, so as to determine the passage from said shortened saidextended configuration, vice versa; where the passage of the activematerial from its extended to its shortened configuration imparts therotor (3) a rotation couple with respect to the stator (2) contrastingthe action of said elastic means (13), and the latter induce a shift ofsaid second portion (12) from said first seating to a followingconsecutive seating (14) of said sequence, during the passage of theactive material from said shortened to said extended configuration.